Gain Partitioning in a Receiver

ABSTRACT

An automatic gain control loop disposed in a receiver is adapted to compensate for varying levels of out of band interference sources by adaptively controlling the gain distribution throughout the receive signal path. One or more intermediate received signal strength indicator (RSSI) detectors are used to determine a corresponding intermediate signal level. The output of each RSSI detector is coupled to an associated comparator that compares the intermediate RSSI value against a corresponding threshold. The take over point (TOP) for gain stages is adjusted based in part on the comparator output values. The TOP for each of a plurality of gain stages may be adjusted in discrete steps or continuously.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and is a continuation of U.S.Nonprovisional application Ser. No. 14/044,817, filed Oct. 2, 2013,entitled “Gain Partitioning in a Receiver”, which is a continuation ofU.S. Nonprovisional application Ser. No. 12/249,269, filed Oct. 10,2008, which application claims benefit under 35 USC 119(e) of U.S.Provisional Application No. 60/979,024, filed Oct. 10, 2007, entitled “ATechnique For Optimizing Gain Partitioning In A Receiver”, the contentsof which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

A receiver system typically consists of a series of stages consisting ofpre-selectivity gain and mixing, frequency selectivity (i.e. a filter)and post-selectivity gain and mixing. Conventional receivers either seta total system gain with a predetermined partition between pre- and postselectivity gain, or rely on a separate controller or demodulator toindependently adjust pre and post selectivity gains to achieve thelinearity/noise tradeoff.

FIG. 1 is a simplified block diagram of a receiver 100, as known in theprior art. In receiver 100, amplifier 110 has a gain G₁ that providespre-selectivity gain. Frequency converter 120, which may be a mixer,provides frequency conversion. Filter D₁ 130 is typically a bandpassfilter adapted to filter out undesired signal. Amplifier 140 has a gainof G2 and provides post-selectivity gain. A local oscillator (not shown)is often used to provide an oscillating signal to frequency converter120. Frequency converter 120, and filter 130 typically have finitelinearity and thus it is desirable to limit the range of signals thatare coupled to them.

FIG. 2A shows a spectrum of exemplary signals received by filter 130.The desired signal is shown as having the frequency Fd. The spectrum ofthe receives signals often includes undesired signal components (alsoreferred to as blockers) shown as having frequencies Fb1 and Fb2 thatinterfere with the desired signal, causing non-linearity, distortion,etc. For example, the spacing and amplitude of the undesired signals Fb1and Fb2 may result in a third order intermodulation distortion productat the output of amplifier 110. As such, it is not desirable to placetoo much gain before filter 130 which is adapted to attenuate theblocker signals, as shown in FIG. 2B. The reduction of the undesiredsignals enables amplifier 140 to amplify the desired frequencies inwithout substantially increasing the amplitudes of the undesiredsignals.

By reducing the gain G1 of amplifier 110, the linearity is improved.Reducing the gain of the first amplifier 110 also reduces the amplitudeof signal S1. To keep the amplitude of signal S4 constant, gain G2 maybe increased. The gain redistribution between amplifiers 110 and 140reduces distortion but also results in degradation of thesignal-to-noise (SNR) ratio. Therefore a tradeoff exists betweenincreasing the gain G1 to improve signal to noise ratio, and degradinglinearity performance of the system (increasing the distortion productsin the signal) when blockers are present.

Gains G₁ and G₂ are typically selected such that the total gain G₁*G₂ isequal to a known value. In accordance with one conventional technique,for a given input signal level S₀, a predetermined gain partitioning ofG₁ and G₂ is used. FIG. 3 is a block diagram of a conventional receiver300 configured to achieve a predetermined gain partitioning of G₁ and G₂using control signal T_(sys). FIG. 4 shown plots of gains G₁, G₂ andG₁*G₂(G_(sys)) for a receiver having predetermined gain partitions.

In receiver 300, the gains of the first and second amplifiers 110 and140, respectively, are controlled by gain controller 310 that controlsthe gains G₁ and G₂ in accordance with an algorithm that provides fixedgain partitioning using signal T_(sys). FIG. 4 shows examples of thegain G₁ from amplifier 110, gain G₂ from amplifier 140 as well as theproducts of these two gains. The attack point (AP) represents the signallevel at which total gain G_(sys) begins to be fall. The take-over point(TOP) represents the signal level at which gain control is passed fromsignal T₂ to signal T₁. The TOP and AP values are typicallypredetermined and fixed. In a typical television system, a demodulatoris used to generate control signals T₁ and T₂.

In accordance with another conventional technique, the output signal ofthe second amplification stage is used to determine the gainpartitioning. FIG. 5 is a simplified block diagram of a receiver 500having gain partitioning controlled by a demodulator 510. Demodulator510 is configured to controls the values of G₁ and G₂ depending on thepresence and level of blockers. Demodulator 510 operates to control thepartitioning of the gain between amplifiers 110 and 140 by sensing theoutput signal S₄ of second amplifier 140. Demodulator 510 may beprogrammed to estimate whether blockers or other undesired signalcomponents are causing distortion in the desired signal. Demodulator 510then reparations the gain by adjusting signals T₁ and T₂.

BRIEF SUMMARY OF THE INVENTION

An automatic gain control loop disposed in a receiver is adapted tocompensate for varying levels of out of band interference sources byadaptively controlling the gain distribution throughout the receivesignal path. One or more intermediate received signal strength indicator(RSSI) detectors are used to determine a corresponding intermediatesignal level. The output of each RSSI detector is coupled to anassociated comparator that compares the intermediate RSSI value againsta corresponding threshold. The take over point (TOP) for gain stages isadjusted based in part on the comparator output values. The TOP for eachof a plurality of gain stages may be adjusted in discrete steps orcontinuously.

In accordance with the present invention, for a given receiver path gaindefined, for example, by the product of the pre and post selectivitygains, the present invention provides a self-contained, compactapparatus and method for adjusting the partitioning between pre andpost-selectivity gain to optimize the signal level entering the filterdisposed in the receiver. The receiver is thus enabled to continuouslytrade off linearity against noise depending on the presence or absenceof undesired signals (blockers) at other frequencies without relying onthe intervention of an external controller or demodulator.

A receiver, in accordance with one embodiment of the present inventionincludes, in part, a first amplification stage, a frequency conversionmodule responsive to the first amplification stage, a filter responsiveto the frequency conversion module, a second amplification stageresponsive to the filter, and a controller adapted to vary a gain ofeach of the first and second amplification stages in response to anoutput signal of the first amplification stage and further in responseto an overall gain selected for the receiver.

A receiver in accordance with another embodiment of the presentinvention includes, in part, a first amplification stage, a frequencyconversion module responsive to the first amplification stage, a filterresponsive to the frequency conversion module, and a secondamplification stage responsive to the filter. The receiver is adapted tovary the gains of the first and second amplification stages in responseto a first and second feedback signals.

In one embodiment, the first and second feedback signals are supplied bya controller responsive to signals representative of the output signalsof the first and second amplification stages. In one embodiment, thecontroller is external to the receiver. In one embodiment, thecontroller is further responsive to the filter. In one embodiment, thereceiver includes a third amplification stage. In such embodiments, thecontroller is further responsive to a third signal representative of theoutput signal of the third amplification stage.

A method of controlling the gain of a receiver, in accordance with oneembodiment of the present invention, includes, in part, amplifying areceived signal to generate a first signal using a first amplificationstage, frequency converting the first signal, filtering the frequencyconverted signal, amplifying the filtered signal to generate a secondsignal using a second amplification stage, and varying a gain of each ofthe first and second amplification stage in response to an output signalof the first amplification stage and further in response to an overallgain selected for the receiver.

A method of controlling the gain of a receiver, in accordance withanother embodiment of the present invention, includes, in part,amplifying a received signal to generate a first amplified signal usinga first amplification stage, frequency converting the first amplifiedsignal, filtering the frequency converted signal, amplifying thefiltered signal to generate a second amplified signal using a secondamplification stage, and varying a gain of each of the first and secondamplification stage in response to first and second feedback signals.

In one embodiment, the method further includes, in part, applyingsignals representative of the first and second amplified signals to acontroller, and generating the first and second feedback signals inresponse to the signals applied to the controller. In one embodiment,the controller is external to the receiver. In one embodiment, themethod further includes applying a signal representative of the filteredsignal to the controller. In one embodiment, the controller is furtherresponsive to a third amplified signal present in the receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of a receiver, as known in theprior art.

FIG. 2A shows a spectrum of exemplary signals received by a filterdisposed in a wireless communication receiver.

FIG. 2B shows the filtering characteristics of a filter adapted toattenuate the undesired signals shown in FIG. 2A.

FIG. 3 is a simplified block diagram of a receiver, as known in theprior art.

FIG. 4 is a simplified gain diagram of an embodiment of amplifier gainsin a system having a predetermined gain partition.

FIG. 5 is a block diagram of a receiver, as known in the prior art.

FIG. 6 is a simplified block diagram of a receiver, in accordance withone exemplary embodiment of the present invention.

FIG. 7 is a simplified block diagram of a receiver, in accordance withanother exemplary embodiment of the present invention.

FIGS. 8A, 8B and 8C are examples of gain plots and gain partitioning forthe receiver of FIG. 7.

FIG. 9 is a flowchart of steps carried out to perform adaptive gainpartitioning, in accordance with one embodiment of the presentinvention.

FIG. 10 is a block diagram of a receiver, in accordance with oneexemplary embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 6 is a block diagram of a receiver 600, in accordance with oneembodiment of the present invention. Receiver 600 is shown as including,in part, amplifiers 110, 140, frequency converter 120, filter 130 andsensor 610. A local oscillator (not shown) provides an oscillatingsignal to frequency converter 120. Frequency converter 120 may be amixer, a multiplier, etc. Demodulator 510 may be external or internal toreceiver 600. Sensor 610 sense signal S1 to determine the strength ofthe RF signal. Signal S1 so sensed is supplied to demodulator/controller510. Also supplied to demodulator/controller 510 is signal S4 that isgenerated by amplifier 140. In response, demodulator/controller 510generates signals T1 and T2 that are respectively applied to amplifiers110 and 140 to control their gains. As see from FIG. 6, receiver 600together with demodulator/controller 510 form a pair of control loops L1and L2, which are independently controlled by the demodulator/controller510. Loop L1 is used to control gain G1 via signal T1, and loop L2 isused to control gain G2 via signal T2. Demodulator/controller 510 mayuse any one of a number of different algorithms to vary the gains ofamplifiers 110, and 140 using signals T1 and T2.

FIG. 7 is a block diagram of a receiver 700, in accordance with anotherembodiment of the present invention. Receiver 700 is similar to receiver600 except that in receiver 700 signal T_(sys) applied to controller 710includes information about the overall gain of the two amplificationstages. Signal T_(sys) may be supplied by, e.g., a demodulator.Accordingly in receiver 700, loop L1 is used to determine G1. Controller710 knowing the overall gain signal represented by signal T_(sys) setsthe proper gain G2 using signal T2. The gain partitioning of receiver700 automatically partitions the gains G1 and G2 to achieve a desiredgain Gsys specified by controller 710 based on input from a singlecontrol line Tsys. Because only one control line Tsys is required inreceiver 700, it is easy to implement. Furthermore, receiver 100 may beconfigured to adapt TOP to trade off linearity with signal to noiseratio depending on the level of blockers. Additionally, controller 710may be exclusive of the demodulator and thus, controller 710 may beimplemented on the same IC as the other elements of the receiver 700.

FIGS. 8A, 8B and 8C illustrates an example of gain curves and gainpartitioning for the variable gain partitioning receiver of FIG. 7. FIG.8A shows the characteristics of the overall gain G_(sys) of receiver700. When signal S1 exceeds a certain reference level, TOP is reduceduntil S1 equals the reference or falls within a certain range of thedesired reference, for example, to TOP₁, as shown in FIG. 8C. When S1falls below the reference, TOP is increased until S1 once again equalsthe reference, for example, to TOP₂, as shown in FIG. 8B.

Referring to FIGS. 6 and 8, controller 710 operates in the followingmanner. Assume that the desired channel signal S_(d) is nearly constant,but blocker levels are fluctuating, causing total signal S₁ to change.When sensor 610 detects that the total signal S₁ has exceeded an optimalreference level, loop L₁ is used to reduce the TOP, effectively reducingG₁ through T₁. G2 is increased through T₂ to maintain a constantG_(sys). Likewise, when sensor 610 detects that S₁ has dropped below thereference level, loop L₁ is used to increase the TOP, effectivelyincreasing G₁ through T₁. G2 is decreased through T₂, again maintainingconstant G_(sys). The optimal reference level varies from application toapplication and can be programmed dynamically as the applicationchanges. Hysteresis may be used to stabilize the circuit in a digitalimplementation.

The receiver 700 of FIG. 7 does not require an external controller ordemodulator to optimize the gain partitioning, making the system verysimple to interface with any demodulator, and any communication standardwithout the need for extensive software development.

A practical digital implementation is presented in conjunction with themethod 900 illustrated below. It provides discrete steps in TOP controland receives a digital S1 signal. A circuit implementing the method 900,such as the controller 710 of FIG. 7, can compare the input S1 level toa reference level and increase or decrease a digital word controllingthe TOP to compensate. The controller circuit can be clocked at a ratethat can depend on the rate that the S1 signal is being updated.

FIG. 9 is a flowchart 900 of steps carried out to perform adaptive gainpartitioning, in accordance with one embodiment of the presentinvention. The process begins at step 910 when S1 (i.e., the outputsignal of the first amplification stage) value after the first gainstage is updated or upon the next iteration of the control loop if theS1 value is continuously updated or updated at a rate faster than therate of the control loop. The controller receives the updated S1 value.

At step 920 a determination is made as to whether the S1 value issubstantially the same as the predetermined reference level REF for theapplication that is presently active. If so, the controller proceeds tostep 930 and determines if the S1 value is less than a predetermined lowreference level REFL. If so, the controller proceeds to step 970 andincreases the Take-Over-Point, up to a predetermined TOP limit.

If at step 930 the controller determines that S1 is not less than thelow reference level REFL, the controller instead proceeds to step 940where the controller determines if S1 is greater than the high referencelevel REFH. If not, the controller proceeds back to step 910 to awaitthe next S1 update without making any changes to the TOP. If, at step940, the controller determines that the RSSI is greater than the highreference level REFH, the controller proceeds to step 960 to decreasethe TOP down to a predetermined lower limit.

Referring to step 920, if the controller determines that S1 is notsubstantially equal to the reference level, the controller proceeds tostep 950 to determine if S1 is greater than the reference level. If so,the controller proceeds to step 970 to increase the TOP, but not toexceed the upper limit. If at step 950 the controller determines that S1is not greater than the reference level, the controller proceeds to step960 to decrease the TOP but not smaller than a lower limit. Thecontroller proceeds from either step 960 or step 970, that is, afteradjusting the TOP, back to step 910 to await the next S1 update.

It is understood that additional signal strength monitoring loops may beadded in the signal path in order to detect which portion of the signalpath is experiencing saturation first. Such capability may be useful forallowing the receiver to distinguish between blockers which are far fromthe desired signal or close to the desired signal.

A close blocker is referred to as an N+/−1 blocker or adjacent channelblocker (that is, a blocker which is one channel above or below thedesired channel N). Blockers further away in frequency are similarlylabeled. In many receivers, an N+/−1 blocker may cause a portion of thesignal path after mixing or filtering to limit receiver performancebefore the mixer saturates. A receiver is more susceptible to N+/−1blockers because the (undesirable) third-order distortion products fromthese blockers are more severe at frequencies closer to the blockers. Toremedy these problems, in accordance with one embodiment of the presentinvention, an adaptive gain partitioning receiver includes sensors inthe signal path to allow the receiver to distinguish between close inblockers, such as N+/1, from N+/−2 and other blockers.

FIG. 10 is a block diagram of a receiver 1000 that includes a pair ofsignal strength sensors. 810 and 820. Receiver 1000 is thus similar toreceiver 700 except that receiver 1000 senses strength of signals S1 andS3. The overall gain of the receiver is defined by signal T_(sys)applied to controller 710. Receiver 1000 thus detects when the weakestlink in the signal path is being strained, and adjusts the gainpartition(s) to relieve the strain on that link. In the N+/1 blockercase, S₃ will reach a level where its distortion from filter D₁ andother baseband circuits will begin to affect the signal before thesignal S₁ becomes the dominant source of distortion. The controller 710can decide to reduce the gain G₁ and compensate by increasing gain G₂,thereby keeping S₃ below a predetermined threshold. Other filters andgain control mechanisms can be introduced in the signal path andcontrolled in a similar manner.

The above embodiments of the present invention are illustrative and notlimiting. Various alternatives and equivalents are possible. Theinvention is not limited by the number of subbands disposed in thediversity receiver. The invention is not limited by the type ofintegrated circuit in which the present disclosure may be disposed. Noris the disclosure limited to any specific type of process technology,e.g., CMOS, Bipolar, or BICMOS that may be used to manufacture thepresent disclosure. Other additions, subtractions or modifications areobvious in view of the present disclosure and are intended to fallwithin the scope of the appended claims.

What is claimed is:
 1. A receiver comprising: a first amplificationstage; a frequency conversion module responsive to the firstamplification stage; a filter responsive to the frequency conversionmodule; a second amplification stage responsive to said filter; and acontroller adapted to vary a gain of each of the first and secondamplification stage in response to an output signal of the firstamplification stage and further in response to an overall gain selectedfor the receiver.
 2. A receiver comprising: a first amplification stage;a frequency conversion module responsive to the first amplificationstage; a filter responsive to the frequency conversion module; a secondamplification stage responsive to said filter; said receiver varyinggains of the first and second amplification stages in response to afirst and second feedback signals.
 3. The receiver of claim 2 whereinsaid first and second feedback signals are supplied by a controllerresponsive to first and second signals representative of output signalsof the first and second amplification stages.
 4. The receiver of claim 3wherein said controller is external to the receiver.
 5. The receiver ofclaim 3 wherein said controller is further responsive to the filter. 6.The receiver of claim 3 further comprising: a third amplification stage,said controller being further responsive to a third signalrepresentative of output signals of the third amplification stage.
 7. Amethod of controlling a gain of a receiver, the method comprising:amplifying a received signal to generate a first signal using a firstamplification stage; frequency converting the first signal; filteringthe frequency converted signal; amplifying the filtered signal togenerate a second signal using a second amplification stage; and varyinga gain of each of the first and second amplification stage in responseto an output signal of the first amplification stage and further inresponse to an overall gain selected for the receiver.
 8. A method ofcontrolling a gain of a receiver, the method comprising: amplifying areceived signal to generate a first amplified signal using a firstamplification stage; frequency converting the first amplified signal;filtering the frequency converted signal; amplifying the filtered signalto generate a second amplified signal using a second amplificationstage; and varying a gain of each of the first and second amplificationstage in response to first and second feedback signals.
 9. The method ofclaim 8 further comprising: applying signals representative of the firstand second amplified signals to a controller; and generating the firstand second feedback signals in response to the signals applied to thecontroller.
 10. The method of claim 9 wherein said controller isexternal to the receiver.
 11. The method of claim 9 further comprising:applying a signal representative of the filtered signal to thecontroller.
 12. The method of claim 9 wherein said controller being isfurther responsive to a third amplified signal present in the receiver.13. A method of gain partitioning, the method comprising: determining areceived signal metric at an intermediate point within a signal path;and adjusting a Take-Over-Point of a gain before the intermediate pointrelative to a gain following the intermediate point based on thereceived signal metric.
 14. An apparatus for gain partitioning, theapparatus comprising: a comparator configured to compare a signal valueagainst a predetermined reference level; and a gain controllerconfigured to adjust an automatic gain control Take-Over-Point based atleast in part on the comparison.